Attentions have been recently focused on electromobiles, hybrid automobiles and fuel cell electric vehicles or the like due to the rising price of petroleum resources and rising international earth environment protection movement, and they have been partially put in practical use. Secondary batteries as an auxiliary power or the like are essential for these driver systems. In addition, high output secondary batteries capable of responding to the sudden starting and sudden acceleration of automobiles are desired. Alternatively, secondary batteries having high energy density are desired from the viewpoint of weight load to the automobiles and enhancement in fuel consumption. Because of these factors, lithium ion secondary batteries having the highest energy density in the secondary batteries and capable of expressing high output have been highly expected.
The lithium ion secondary battery, which uses an electrolysis solution containing a nonaqueous solvent containing lithium salts, has a structure where a positive electrode and a negative electrode are separated by a separator, the positive electrode provided with a positive electrode active material, the negative electrode provided with a negative electrode active material. Alternatively, since the conductivity of the positive electrode active material itself in the positive electrode is low, conductive materials such as carbon black are added in order to enhance the conductivity.
The above positive electrodes are generally produced by applying a slurry obtained by mixing active materials such as LiMn2O4, conductive materials such as carbon black, a binder, and a solvent onto a metallic foil as a current collector and drying the slurry. As a result, the fine structure of the positive electrode has a structure where particles made of a positive electrode active material having low conductivity and particles made of a conductive material having a particle diameter smaller than that of the particles made of the positive electrode active material are dispersed and combined.
In the positive electrode of the lithium ion secondary battery, lithium is stored in the positive electrode active material in discharging. In that case, electric discharge is advanced by the operation of lithium ions diffused to the positive electrode side and electrons electrically conducted from a positive electrode current collector. Alternatively, the electrons and ionized lithium are emitted from the positive electrode active material in charging. Therefore, the selection of a conductive material having high conductivity and the fine composite structure of the positive electrode active material and conductive material as factors which affect the characteristics, particularly high-rate discharge characteristics (high output) of the battery are very important.
For such reasons, some enhancements of the fine composite structure for the positive electrode have been attempted. For example, Patent Document 1 proposes a positive electrode material in which the surface of the positive electrode active material is covered with a conductive material in a covering rate of 15% or more by a process for mixing the positive electrode active material with the conductive material to apply a compression shearing stress in a dry state. Alternatively, Patent Document 1 discloses that graphite having a particle diameter of 1 to 20 μm is added when a positive electrode is produced using the positive electrode material.
Alternatively, a method for producing a positive electrode composite material by wet-mixing has been also known. For example, Patent Document 2 proposes a producing method for wet-mixing ferrous phosphate hydrate, phosphate lithium and a carbonaceous material precursor, removing a solvent to obtain a mixture, and grinding and firing the mixture to produce a carbon composite material.
Alternatively, Patent Document 3 attempts the improvement of a conductive path by adding carbon fiber into a positive electrode active material.
Furthermore, Patent Document 4 proposes a positive electrode formed of a composite material obtained by adding both carbon black and carbon fiber into a positive electrode active material and mixing the carbon fiber, the carbon black and the positive electrode active material.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-14519    [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-292309    [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-103392    [Patent Document 4] Japanese Patent Application Laid-Open No. 2004-179019